Sounds like the ground is locked on, so it must be some sort of short unless something's mis-wired. Have a look at the under playfield mounted transistor and make sure there's no short between the legs (that go through the metal bracket) and the bracket itself.

There is no short between the transistor legs and the metal bracket.

I thought from the start that it might be a ground fault, but even unpicking the harness I can't find anything.

Game works fine if I disable the k target reset solenoid.